Linear synchronous motors are used in practice as drives for a wide variety of applications and in a wide range of different dimensions. Aside from linearly driven advancing devices in machine tools or positioning systems, linear synchronous motors are also used for transportation systems such as magnetic levitation railroads.
In particular, in the case of short travels, use is made of short-stator linear motors in which the travel path, as the positionally fixed component, is equipped with magnets of alternating polarity. The primary part of the motor is then formed by drive windings of the moving component, for example the vehicle that is moved along a travel path. Short-stator linear motors are commonly used in linear drives in machine tools, wherein the alternating field provided for propulsion can be supplied to the moving component for example by means of a movable cable. A short-stator linear motor is known for example from EP 1 056 187 A1. In the case of short-stator linear motors that have only a short travel, the positionally fixed component that is equipped with magnets is commonly manufactured as a single coherent part.
If, however, it is sought to cover relatively great distances at high speeds, use is made in practice of elongate-stator synchronous motors. For example, a linear synchronous motor having an elongate stator which has coil windings and having a runner is known from the periodical ZEVrail, special edition October 2003, pages 10 to 16. In the case of an elongate-stator synchronous motor of said type, the elongate stator, as the positionally fixed component, is manufactured in parts and then installed. Even though the arrangement of said elongate-stator sections is performed with very high accuracy, local discontinuities are unavoidable over the entire motor distance. Accordingly, gaps are provided between the elongate-stator sections arranged in succession in the longitudinal direction, which gaps are necessary for the installation, maintenance and exchange of the elongate-stator sections and in order to make it possible for expansions and deformations, which may for example be attributable to a thermal change in length or a movement of the supporting structure, to be compensated. The gaps give rise to a deviation in relation to the otherwise equidistant arrangement of the coil windings of the elongate stator, which deviation leads to force fluctuations during the operation of the linear synchronous motor, this also being referred to as a force undulation. Such force fluctuations may, in the form of shocks or vibrations, lead to considerable losses in comfort, wherein increased mechanical and electrical loading of the motor also arise.
To minimize such disturbances, it is attempted to keep the gaps between successive elongate-stator sections as small as possible through an optimization of the construction. The described force fluctuations however cannot be fully eliminated even with relatively great effort with regard to design.
WO 2009/146821 A1 is concerned with the reduction of force fluctuations in a linear synchronous motor, wherein electronic control is proposed for compensation purposes. By means of such electronic compensation, the force fluctuations based on the geometry of the linear synchronous motor can be suppressed, as a result of which the geometry can be maintained unchanged.
U.S. Pat. No. 3,712,240 A describes an asynchronous motor of a different generic type, in which simple conductive plates, for example aluminum plates, are provided as a secondary element. To permit a uniform drive action, it is the intention for the degree of overlap between primary elements and secondary elements to always be constant. In U.S. Pat. No. 3,712,240, no magnets are provided, such that the problem of drops in force caused by individual magnets does not arise.